1. Field of the Invention
The invention relates generally to electrical solenoids that produce a linear, axial force and more specifically to that class of electrical solenoids known as force motors which produce a relatively short displacement which is proportional to a driving current.
2. Description of the Prior Art
Solenoids are generally characterized by an actuation direction which does not change with regard to the direction of the energizing current. In other words, if a direct current supply has its polarity reversed, the solenoid still provides axial movement in the same direction.
Force motors are distinguished from solenoids in that they use a permanent magnet field to prebias the airgap of a solenoid such that movement of the armature of the force motor is dictated by the direction of current in the coil. Reversal of the polarity of current flow will reverse the direction of the force motor armature displacement.
Force motors are frequently used to drive a valve spool in a high performance aircraft where efficiencies of weight, size, cost and power consumption are of prime consideration. It is therefore advantageous to minimize losses associated with producing high magnetic forces and to minimize the size of the permanent magnets which normally have densities and relative costs higher than the solenoid iron.
FIG. 1 in the present application illustrates a conventional force motor with a simplified construction for ease of explanation. A stator 10 includes mounting brackets 12 and an iron core which provides a path for flux travel. The armature 14 is mounted on and moves with output shaft 16. Included in the stator mount is magnet 18 which generates a flux flow through the stator and the armature as indicated by the solid line arrows 20. This flux from magnet 18 travels in opposite directions across airgaps 22 and 24. Coils 26 and 28 are provided and are wound so as to provide flux flow paths indicated by dotted line arrows 30 which cross airgaps 22 and 24 in the same direction. Obviously if the current flow in coils 26 and 28 were reversed the direction of the coil generated flux flow paths shown by dotted line arrows 30 would be reversed for both airgaps 22 and 24. It is noted that the permanent magnet 18 can be mounted in the stator assembly, as shown, or may be part of the armature.
Operation of the prior art force motor provides an output movement by shaft 16 when current in one direction is provided to coils 26 and 28 and movement of the output shaft in the opposite direction when the opposite current flow is provided to coils 26 and 28. This movement direction is caused by the fact that, as shown in FIG. 1, flux flow generated by the permanent magnet 18 (shown by solid line arrows 20) is in the same direction as coil generated flux flow (indicated by dotted line arrows 30) across airgap 22 but in an opposite direction across airgap 24. This causes a greater attraction at airgap 22 than would exist at airgap 24 and thus the armature is attracted towards the left hand stator portion moving the output shaft to the left.
If the coil generated flux flow were reversed (by winding the coil differently or merely reversing the polarity of the direct current supply) the flux flow would be cumulative across airgap 24 and differential across airgap 22 resulting in the armature movement to the right and consequent output shaft movement to the right. Airgaps 22 and 24 are designated working airgaps in which the flux passes through an airgap and, as a result, generates an attractive force between the stator and armature which is in the axial direction. The prior art force motors have an additional airgap 32 which may be characterized as a non-working airgap as flux flow is in the radial direction and thus even though there is an attraction between the stator and armature, this does not result in any increase in force in the axial or operational direction of the force motor. In order to maximize flux flow (minimizing airgaps) this dimension is made as small as possible (minimizing reluctance of the flux flow path) although a sufficient clearance must be maintained to allow for relative movement between the stator and armature.
It will be further recognized by those familiar with the utilization of permanent magnets in force motors that the magnet will have a preferred optimum energy product point on its de-magnetization curve about which the magnet should operate for maximum efficiency. The closer the magnet operates to this point, the smaller the magnet can be. Further, the magnet length, cross sectional area and strength are dictated by the level of flux required to drive through the magnetic circuit to achieve the desired performance of the force motor. Thus, force motors having a high force requirement typically have a low reluctance magnetic path due to the cross sectional area of the iron necessary for producing high forces and a relatively large volume of permanent magnets to produce the necessary airgap flux. Of course, attendant with the desired high flux level of a low reluctance magnetic circuit are losses which may be expressed in ampere-turns in the iron and also in the non-working airgap(s) which further detract from the efficiency of the motor. These losses are accounted for by increases in the electrical power source and/or the requirement of a larger permanent magnet than would otherwise be necessary.